1. Field of the Invention
The present invention relates to a process for the removal of dimethyl ether and/or methanol from C4 hydrocarbon streams.
2. Related Art
The production of methyl tertiary butyl ether (MTBE) is usually carried out by selectively reacting isobutene contained in a refinery C.sub.4 stream with methanol in the presence of an acid catalyst. The resulting MTBE product is separated from the C.sub.4 stream which is then potentially useful for other purposes such as alkylation. However the C.sub.4 stream will contain dimethyl ether (DME) and/or methanol in small amounts generally from 10 to 50 ppm up to several thousand parts per million. The precise contaminant and its amount will vary somewhat depending on the specific MTBE process employed.
The separation of MTBE from the hydrocarbon product stream is not a problem because of the boiling point difference between MTBE and C.sub.4, the DME and methanol are lower boiling and contaminate the C.sub.4 fraction.
In acid alkylations the presence of DME causes at the least the use of more acid than the alkylation per se. In the case of sulfuric acid alkylations this may be a tolerable detriment, however, in the case of HF alkylations an acid soluble oil is formed which interferes with the separation of the alkylate product from the acid and may lead to fouling of the alkylation unit.
The removal of methanol can be carried out with a water wash system since methanol is relatively soluble in water, however, conventional water wash systems when used to treat the entire residual hydrocarbon stream do not remove the major portion of the dimethyl ether. In order to remove the DME to acceptable levels, the example for alkylation, would require a volume of water equal to about three times the volume of the hydrocarbon stream being treated. Hence, a large volume of waste water would be produced, which in addition to DME would contain large amounts of hydrocarbon. Handling this volume of waste water treatment makes the conventional water wash unattractive.
The present process provides for the removal of DME and methanol to satisfactorily low levels while using very low volumes of treatment water. In one embodiment a close water wash system which substantially eliminates waste water disposal is employed. These and other features and advantages will become apparent from the following description.